Wheatstone Bridge Principle:

A Wheatstone bridge show in the figure is employed to measure the resistance of an electronic conductor.  It works on the principle of obtaining balance between two arms with the help of a balance indicator (e.g. a galvanometer) at the condition of potential being equal.

Let R_x be an unknown resistor, R₁ and R₂ two standard resistors, R₃ an adjustable resistor and G a galvanometer.  The bridge is linked to a source of power S, a battery, and a tapping key K is placed in the path to control the connections.

Figure: A DC Wheatstone bridge circuit